Illuminated signboards have been used for many years to advertise the names of stores and businesses because of their high visibility both during the day or at night. The simplest such signs typically comprise a frame containing a light source, which may be incandescent, or fluorescent, and a translucent panel bearing the printed information mounted in front of the light source. The translucent panel is typically rigid and made from ground glass or plastic materials such as high impact polystyrene, poly(methylmethacrylate) sheeting, etc., and the image is formed by paint or screen printed ink directly onto the panel. Such signs are more or less permanent, and usually remain in place for many years. It is important that the panel be translucent rather than transparent, so that the back of the frame and the light source itself is not visible through the unpainted or unprinted areas.
More recently, illuminated signs have been used for general advertising which require more complex images of higher visual impact, and which usually require that the content be changed from time to time. Such signs have become widely used in many public places both indoors and outdoors such as airports, rail and bus stations, hotel lobbys, theaters, pedestrian under-passes, etc. This type of illuminated sign is often referred to as a "backlit display", and there are many designs used to achieve the purpose of the display. However, a commonly used display consists of a box containing a light source at the back and a frame in front which holds the image. The frame may be made from glass or plastic as before, but in this case the material of the frame is transparent and the image is printed on a translucent substrate which is flexible in order to allow easier installation and removal. Using a flexible image media also allows a greater choice of cheaper imaging processes.
The degree of light transmission of the image media is important because if the material is too opaque, the image will appear dull and the colors will lose their vibrancy. At the same time, the light from the incandescent bulbs or fluorescent tubes in the back of the display must be highly diffused as it passes through the media or the illumination of the sign will not be uniform. When this happens, the areas of the display directly in front of the light source(s) appear brighter than those further away, and the overall image has an objectionable appearance. Such bright areas are sometimes called "hot spots" and avoiding them often requires that the media must have lower light transmission. Clearly, the degree of light transmission must be very uniform so that the strength of illumination of the image is the same at all points over the area of the display.
For low to moderate quality backlit display applications, the image substrate is usually a thin paper stock. The fibrous structure of the paper provides a means to diffuse the light, but a thin paper is needed to provide sufficient light transmission. The image is formed on the paper by conventional gravure printing or offset lithographic printing or by other known printing processes. This type of media has the advantages of low cost and the ability to produce many copies easily, but quality is always limited by the non-homogeneous composition of the paper. The amount of light passing through the paper varies considerably from point to point as the thickness of the paper and the local density of pulp fibers always varies. This leads to a mottled effect in solid print areas and greatly limits the quality of the image which can be produced using paper in a backlit display. Paper also has inherent problems of easy tearing, damage by moisture, dimensional change under different humidity conditions and yellowing.
For high quality images, photographic processes have been used to produce images in a photoemulsion coated on a plastic substrate, usually polyethylene terephthalate (PET) film. This is very costly compared to conventional printing on paper, but image quality and visual impact can be extremely high and the imaging media based on PET film is much more durable than paper. Recently, wide format inkjet printing has come to be widely used as a lower cost alternative to the photographic process for producing high quality images for backlit display, and again the printing medium is typically a polyester film bearing a coating to absorb the water based inkjet ink. Recent advances in inkjet printing technology have allowed the image quality produced to rival that of the photographic process, especially in applications such as backlit displays where the image is typically viewed from a distance of several feet.
Until now, in both photographic and inkjet printed backlit signs, the required translucency of the imaging media has been achieved by coating the transparent PET film substrate with a light diffusing coating, usually a mixture of a resin or gelatin binder and a finely dispersed pigment, such as titanium dioxide. This light diffusing coating is usually applied to the reverse side of the film from the imaging side. In both photographic and inkjet media, a coating has to be applied to the reverse side of the film anyway to control curl of the media which would otherwise occur during printing/processing due to hygroscopic expansion of the image coating, so the light diffusing coating does not unnecessarily complicate the structure of the final product. The same coating can be used on transparent PET films of different thickness so that the backlit display media can easily be produced in a range of thicknesses, all having the same light transmission properties.
While satisfying many of the requirements of backlit displays, this diffuser coating approach has a number of inherent disadvantages. As the light transmission of the media must be very uniform for high quality images, the thickness of the light diffuser coating must also be very uniform from point to point. Any defects in the light diffuser coating are strongly highlighted under backlit display conditions, so the quality requirements of this coating must be extremely high. Accordingly, production efficiencies for light diffuser coatings are often much less than for clear coatings so that production costs for backlit display media are relatively high. Additionally, advances in inkjet receiver coatings have produced many systems today which resist curling without the use of any backcoating, but these simpler structures with their lower production costs offer no advantage in backlit display films because of the need for light diffusion. A final problem with the light diffusion coatings is that unless they are very hard and well adhered to the substrate PET film, they can be easily damaged by handling after printing or during installation. Even a light scratch will show up strongly under backlit conditions, especially if it occurs in a dark area of the image. Re-work at this final stage of backlit display production is extremely costly.
Accordingly, an object of the present invention is to provide a polyester composite film for backlit display media which has the necessary light diffusion properties built directly into the film. This eliminates the need to apply a costly light diffuser coating to the film and reduces overall production costs accordingly. Incorporation of a light diffusing filler directly into the coextruded polyester composite film not only allows very uniform light diffusing properties to be achieved, but also results in a much more durable structure with less pin-holing than using conventional coatings. Such a composite film advantageously reduces re-work for backlit display printers and installers.
U.S. Pat. No. 5,680,720, issued to Asazuma et al on Oct. 28, 1997, discloses a semi-transparent laminated film for use in an illuminated signboard comprising a biaxially stretched thermoplastic resin film substrate and a monoaxially stretched thermoplastic film, preferably a polyolefin copolymer film, containing from 0.3 to 5% by weight of a reflective white powder such as titanium dioxide and zinc oxide. The incorporation of titanium dioxide and zinc oxide into the film causes an increase in the opacity of the film accompanied by a drop in light transmittance. In contrast, the present invention incorporates silica particles in the film having an optimum particle size to enhance light scattering. Most of the light is scattered forward, i.e. it contributes to the brightness of the display image as contrasted to titanium dioxide which scatters tip to 50% of the light backwards and therefore wastes this portion of the light.